1. Field of the Invention
The invention relates to an all-solid lithium secondary battery in which a reaction between an electrolyte-containing layer including a sulfide-based solid electrolyte material and moisture contained in an external air can be inhibited and water resistance is increased.
2. Description of the Related Art
Following rapid spread of information device and communication devices such as personal computers, video cameras, and cellular phones in recent years, great importance was placed on the development of secondary batteries, for example, lithium secondary batteries that excel as power sources for such devices. In the fields other than those of the information-related devices and communication-related devices, for example, in the field of automotive industry, the development of high-output and high-capacity lithium secondary batteries was advanced for electric automobiles and hybrid automobiles as vehicles with a low environmental impact.
However, lithium secondary batteries that are presently available on the market use organic electrolytes including combustible organic solvents. As a result, it is necessary to install safety devices that inhibit temperature increase during short circuit and improve the structure and materials for preventing short circuiting.
By contrast, all-solid lithium secondary batteries in which a liquid electrolyte is replaced with a solid electrolyte and which have an all-solid structure use no combustible organic solvent inside the battery. As a result, the safety device can be simplified and the batteries excel in production cost and productivity
The all-solid lithium secondary battery is formed, for example, by configuring a pellet having a three-layer configuration including a positive electrode, a solid electrolyte, and a negative electrode by a powder molding method, introducing the pellet into a conventional coin-type battery case or button-type battery case, and sealing the outer circumference of the case. In such an all-solid lithium secondary battery, the battery assembly group composed of the positive electrode, negative electrode, and electrolyte is an entirely solid body. As a result, electrochemical resistance of such a battery tends to increase and output current tends to decrease by comparison with those of lithium secondary batteries using an organic electrolyte.
Accordingly, in order to increase the output current of the all-solid lithium secondary battery, it is desirable to use an electrolyte with a high ion conductivity. Sulfide glass such as Li2S—SiS2, Li2S—B2S3, and Li2S—P2S5 demonstrate a high ion conductivity in excess of 10−4 S/cm. Furthermore, a high ion conductivity of about 10−3 S/cm can be obtained by adding LiI, Li3PO4 or the like. In glass based on these sulfides, polarization of sulfide ions is higher than that of oxide ions and electrostatic attraction with lithium ions is small, which is apparently why such glass demonstrates ion conductivity higher than that of oxide glass.
However, in a battery using a solid electrolyte material including the sulfides as the main components (sulfide-based solid electrolyte material), the sulfide-based solid electrolyte material has low water resistance and easily reacts with moisture. As a result, the sulfide-based solid electrolyte material easily deteriorates with generation of hydrogen sulfide.
As a method for inhibiting the deterioration of the sulfide-based solid electrolyte material caused by such reaction with moisture, for example, Japanese Patent Application Publication No. 8-167425 (JP-A-8-167425) discloses a manufacturing method by which a sulfide-based solid battery is assembled under a regenerated argon gas atmosphere from which moisture and/or oxygen has been removed. However, a problem associated with the sulfide-based solid battery obtained by such a method is that when the battery is used in an environment in which moisture is present, such as the atmosphere, the reaction of the sulfide-based solid electrolyte material with the moisture contained in the external air such as the atmosphere cannot be inhibited.